Feather vane and down have been subjected to chemical and enzymic treatment and specimens obtained in this way examined in the electron microscope. It has been found that the main body of the feather parts is enveloped by a cuticular sheath of a somewhat different character. The cuticle can be divided into at least two different layers, the outer being probably of non-protein composition, and the inner a more resistant keratin layer. They are called epi- and exocuticle, in view of the close relationship with corresponding layers in the cuticle of wool and other animal hairs. It seems probable that the epicuticle is responsible for such properties as water repellency, low friction and the chemical inertness of the surface. In parts of the vane the surface is covered with small rods (length around 1 micron). These micro-horns give a velvet-like appearance to the surface, which might be helpful in increasing the water-repellency and in reducing the mobility of air, thus increasing the carrying power during flight and the heat insulation. They may also give the feather parts frictional properties which favour suitable relative positions of the feathers.